Recent years have marked exceptional scientific breakthroughs in our understanding of inherited susceptibility to breast cancer, with the identifications of the BRCA1 and BRCA2 genes. Inherited mutation of one of these genes make their carriers at a much increased risk of developing breast as well as ovarian cancer. It has become possible, and increasingly common, to test for the presence of these genes. While the availability of testing represents a major opportunity both scientifically and clinically, it also creates important challenges for women facing the testing decision and for their physicians and genetic counselors. The chance of carrying a genetic mutation caries markedly from women to woman, depending on family history of breast and related cancers. As a result, increasing attention needs to be given to understanding and conveying risk information in an individualized way. Evidence is emerging that the simple rules that provided important initial guidance in this process are not fully adequate, and that computer-based risk prediction algorithms that fully explicitate our knowledge about the nature of the inheritance mechanism can contribute importantly to a woman's decision. In the first phase of the proposed research we will further develop a computer-based risk prediction algorithms for determining the probability that an individual is a carrier of a mutation of the BRCA1 and BRCA2 genes based on a detailed description of his or her family history. In the second phase of the proposed research we will validating the model using data arising in high risk clinics, that is in the same conditions as those of the intended use of the model. In view of the national shortage of trained genetic counselors, and the great public interest in and awareness of genetic testing, findings from our study will make a significant contribution to understanding and promoting informed decision-making about genetic testing for susceptibility to breast cancer.